This invention relates to a device for measuring the convergence of three electron beams of a color cathode ray tube (referred to as CRT hereinafter), i.e., red beam, green beam, and blue beam.
Conventional convergence measurement devices for color CRT require two times or more of image pickup. For example, in a device of Japanese Unexamined Patent Publication No. 59-74780, all the phosphor dots of a viewing screen of a color CRT are firstly glowed at the same time to produce an entire white pattern. The pattern is picked up with changing color filters by a color filter changing device so that respective positions of red, green, and blue phosphor dots are specified and then put into memory.
Next, a measurement pattern is glowed in which centers of red, green, and blue glow images coincide with one another. The measurement pattern is picked up without color filters to obtain positional information of the measurement pattern in the form of a group of glowing phosphor dots. Positions of the glowing phosphor dots of the measurement pattern are compared with the specified positions of red, green, and blue phosphor dots to obtain a measurement pattern of each color of blue, green, and red which is identical to a measurement pattern picked up in each color. The luminous center of gravity of the measurement pattern of each color are then calculated to find any misconvergence.
It would be seen that the above-mentioned device requires four cycles of image pickup, specifically, three cycles of an image pickup in order to specify positions of phosphor dots, and one cycle in order to obtain positional information of a measurement pattern, consequently requiring a longer measuring time. Also, it will be seen that the conventional device carrying a color filter changing device requires mechanical operation, which consequently causes the maintenance to be cumbersome and also the size larger.
It could be understood that if using a color image pickup device having color filters, it may be eliminated to additional provide a color filter changing device. Also, it could be understood that convergence measurement may be completed at only one time of image pickup because signals for each color are obtainable at the same time. However, the following problems remain.
As shown in FIG. 2, phosphor dots applied on the viewing screen of a color CRT have a wide range of frequency in respect of emission characteristics. As shown in FIG. 3, color filters used in a color image pickup device have a wide range of frequency in respect to spectral characteristics. With FIG. 2, for example, blue phosphor dots are glowed in a wave length range of about 400 nm to about 540 nm. When a color image pickup device receives light from the glow blue phosphor dots, blue picture elements (b) of the pickup device, being covered with blue filters, generate a great magnitude of output because the blue filter permits a large amount of the light of 400 nm to 540 nm to pass therethrough as shown in FIG. 3.
Green picture elements (g) of the pickup device are covered with green filters which permit light of 450 nm to 540 nm to pass therethrough. Consequently, the picture elements (g) generate a perceivable magnitude of output.
Red picture elements (r) of the pickup device are covered with red filters which permit light of 540 nm or more to pass therethrough but permit a smaller amount of light to pass than the green filter (g). Consequently, the red picture elements (r) generate a smaller magnitude of output.
As mentioned above, even when one kind of electron beam is irradiated, the blue, green, and red picture elements of the image pickup device generate their respective signals at the same time. Accordingly, when convergence measurement is carried out based on a white pattern which is produced by putting the three electron beams into work at the same time, it is very difficult to discriminate which of the three electron beams causes the detected signals. Consequently, an accurate convergence can not be obtained.